[sdiy] Waveforms (was dx, chorus and Spock)

jhaible jhaible at debitel.net
Fri Aug 16 22:26:13 CEST 2002

Hi Gene,

found your name on the CD sleeve - congrats!

> In other words are the spectral spikes sharp and distinct and highly
> differentiated, and if the waveform is somehow warped they are "smeared"
> "blurred"? I do realize that if every cycle of the waveform is identical
> the previous one and the following one, then the harmonics should fall on
> integer ratios only (right?).

You only get "sharp spectral spikes" when the signal is unmodulated - and
when it's switched on for a long time (strictly speaking for an infinite
Unmodulated and gated already means a sinx/x function instead of spikes.
But I'm not sure if this really matters. I think there is a more
approach to animated sounds vs. unmodulated sounds than the widening
of the furier spectrum: The animated sound is "more interesting". Especially
when beating from several sources is involved, when frequency fluctuations
are transformed to amplitude modulations. "Several Sources" does not
even necessarily mean "several VCOs": If you're playing into a Room (or
a reverb box), frequency modulations from a single VCO are enough for this
effect - the direct signal interacts with the reflections.

> Hey - I wonder what a good spectrum analyzer goes for on ebay. Does
> know of a good make/model with a comfortable audio-range display?

I don't know - but you will have to make very long measurements (slow sweep)
you will catch the "sharpness" the spectral spikes.

In your original experiment the filter might have had more contribution to
effect than your VCO. The ASM has amplitude limiting diodes like the
SEM filter, doesn't it? (I have not checked - please correct me if I'm
This makes the filter "more musical" but "less analytical" than certain
filters. For instance, comparing an Emu UAF and the Oberheim SEM filter
(both State Variable types), the Emu is much better in picking out overtones
(much higher maximum Q than the Oberheim), but the SEM filter sounds
much smoother when the headroom is exceeded.


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